Power tongs have been in existence for many years and are generally employed in the oil and gas industry to grip and rotate tubular members, such as drill pipe. It is necessary to grip drill pipe with high compressive forces while applying a high degree of torque in order to break apart or tighten threaded pipe connections. An example of a conventional power tong can be seen in U.S. Pat. No. 4,084,453 to Eckel. Most current power tong designs such as Eckel include an open slot or throat, through which the drill pipe is passed in order to place the power tong in position around the pipe. Typically power tong designs employ a cam mechanism for converting a portion of the torque into a gripping (compressive) force normal to the pipe. This conversion is often accomplished utilizing a power-driven ring gear having an interior cam surface. The ring gear will also have an opening corresponding in size to the throat of the power tong. The ring gear is rotatively positioned between an upper and a lower cage plate which form a jaw carrier. The cage plates are also rotatively positioned in the power tongs and also have openings corresponding with the throat of the power tong. Multiple jaw members are typically secured between the upper and lower cage plates in such a manner that relative movement between the ring gear and cage plates causes the jaws to ride upon the cam surfaces and to close on the drill pipe. After the jaws have closed on the drill pipe, the cage plates and the ring gear will then rotate as a unit to apply torque to the drill pipe.
In order to initially hold the cages plates stationary while the ring gear rotates sufficiently to close the jaws, a brake band typically encircles the upper cage plate. The brake band applies an initial frictional force to the cage plate, holding the cage plates stationary and allowing the ring gear to move relative to the cage plates. Generally, the brake band is adjustable such that it may be tightened or loosened in order to vary the amount of frictional force applied to the cage plate. After relative rotation begins and the jaws mount the cam surfaces and close on the drill pipe, the jaws will begin to transfer torque to the cage plates which will eventually overcome the resisting frictional force of the brake band. Because the cam surfaces translate torque into radial force, a higher torque needed to overcome the resistance of the brake band will result in a higher radial force being placed on the drill pipe. Therefore the frictional resistance of the brake band may be adjusted to regulate the radial load placed on the drill pipe.
While the prior art brake band has accomplished its intended function, it is very inefficient. After the cage plates and ring gear begin moving as a unit to rotate the drill pipe, the friction caused by the brake band now provides undesirable resistance to the torque being applied to the drill pipe and the energy expended by the power tongs in overcoming this resistance is wasted. Also, since it is primarily the resistance of the brake band which determines the radial force with which the jaws grip the drill pipe, this force cannot be varied without stopping work to adjust the brake band. Furthermore, it is difficult to insure the brake band will provide a constant resisting force to the cage plate. For example, grease or a similar substance may be deposited between the cage plate and brake band thereby lowering the frictional resistance of the brake band. Additionally, brake bands wear over time and do not grip as tightly as originally designed. This wearing tendency is often caused by the fact that the cage plate does not present a continuous surface but has an opening corresponding to the throat of the power tong. The contact of this opening with the brake band during rotation of the cage plate causes excessive wear and damage to the brake band.
After the desired torque has been applied to a drill pipe joint, the direction of the ring gear's rotation is reversed to allow the jaws to back off of the cam surfaces and release the pipe. Again the brake band holds the cage plate stationary during the ring gear's initial reverse rotation. Once the jaws have backed off the cam surfaces, it is necessary for the ring gear and cage plates to move in unison to align the openings of the cage plates and the ring gear with the throat of the power tongs. This is accomplished by the interaction of a backing lug on the ring gear and a backing pin inserted into the upper cage plate. The upper cage plate will have two apertures for receiving the backing pin. The backing pin is inserted into one of the two apertures depending on which direction the ring gear will be applying torque to the drill pipe. The ring gear will rotate relative to the cage plates until the backing pin contacts the backing lug. At this point, the openings of the ring gear and cage plates will be aligned and the interaction of the backing pin and backing lug will cause the ring gear and cage plates to rotate together. The operator of the power tong continues to rotate the ring gear in order to observe when the openings of the ring gear and cage plates are aligned with the throat of the power tongs. This alignment is necessary to back the power tongs off of the drill pipe.
This leads to another disadvantage encountered in prior art power tongs when the tong operator desires to reverse the rotational direction of the power tong. To do so, the operator must manually switch the backing pin to the alternate aperture. This may be very difficult in certain drilling operation, such as when operations are being carried out by an automated pipe handling system and the operator controlling the power tong is working from a control station some distance from the tong. The remote controlling of power tongs presents another problem in that the operator may not be able to view the throat of the power tong from his location. Thus the operator is not able to visually align the openings of the ring gear and cage plates with the power tong's throat in order to back the power tong off of a drill pipe.